The weight may be the same, but the air incorporated into the WOTA's spinning process does not weigh anything. It might be worth a gauge swatch, because you really can't tell just looking at a yarn if doubling will equal something else. Here's a picture of the two. This is what I mean about the fluffiness of WOTA.

Yes and as a Math major I can attest that Density = Mass/Volume
so even through the Mass (weight) is the same, the Volume (space they take up) is not - Ingrid makes a good point about the air - that's what's changing the volume and density

Does anyone else feel like they should be wearing a white lab coat? (hand knit of course)